Insulating walls from inside: dry lining, thickness and condensation

Anyone looking up how to insulate walls from the inside can almost never choose to insulate from the outside: they live in a flat, the facade belongs to the comunidad de propietarios (the building’s community of owners), or they are only worried about the north-facing room. Insulating from inside is what is in their hands. It works, but it comes at a cost the videos do not show: the centimetres it eats up and the cold it leaves behind.

To explain it we went to the technical data sheet of a current dry lining panel and to the document the CTE, the Spanish Building Code (Código Técnico de la Edificación), uses to check whether a wall will suffer condensation. What follows comes from there.

What insulating from inside means

It means lining the inner face of the wall with a layer of insulation and finishing it with a board that can be painted. The most direct way is a panel that already has both together. Its data sheet describes it as a panel “made up of a Gypsum Plasterboard” plus “expanded polystyrene”, and its application, in its own words, is the “Construction of direct dry linings”, stuck to the wall with a bonding adhesive.

There are also dry linings fixed to a frame of metal studs, set away from the wall. Here we stick to the direct-bonded kind, which is the one on the data sheet and the one of the two that needs less space.

What it costs in centimetres

The panel on the data sheet has a 12.5 mm board and 60 mm of insulation. Together that is 72.5 mm, just over seven centimetres, and on top of that comes the adhesive it is stuck on with. That is how far the wall moves into the room, along its whole length.

It does not sound like much until you think about the skirting, the socket that has to be brought forward, the window frame that ends up more deeply recessed and the built-in wardrobe that no longer closes. That is why it is almost always the external wall that gets insulated, not all of them.

Where the cold ends up

This is the part that is hard to believe. The Building Code works out the temperature inside a wall layer by layer, and each layer takes a share of the difference between outside and inside in proportion to its thermal resistance. An insulation board has far more resistance than brick, so when it goes on the inside, it takes almost all of the drop. The wall behind it gets colder.

And that is where vapour comes in. The air in the house carries water, which tries to escape outwards through the wall. If it finds a point cold enough, it condenses. In each layer the document compares the vapour pressure with the saturation pressure for its temperature, and for cases where a barrier is needed it says so plainly: the vapour barrier “must be placed on the warm side of the wall, taking care during installation to avoid any tears or damage to it”.

This is not an academic detail. The same document warns that, “as a general rule and unless expressly justified, it is not advisable to accept the presence of condensed water in insulating materials”. That is why a well-made dry lining is either calculated or fitted with a barrier.

What the wall gains on the inside

There is one advantage you notice straight away: the inner surface is warmer. The Building Code expresses this with the temperature factor of the inner surface, which for a wall is calculated as 1 minus its thermal transmittance times 0.25: the better the wall insulates, the closer its inner face gets to the temperature of the room. A warmer wall is a more comfortable wall and one where mould has a harder time. Why mould goes looking for exactly the cold surfaces: anti-mould paint and its limits.

What dry lining cannot cover are the points where the facade is interrupted on the inside: the edge of the floor slab at floor and ceiling, the columns, the area around the windows. They are still the coldest part of the room. That is one of the three reasons why insulating from the outside performs better, and we will not repeat it here.

On the coast: humid air indoors

In Málaga the winter is mild: the document’s climate table gives an average of 12.2 °C in January, with 71% relative humidity. With 20 °C inside, the difference is small compared with a city inland, and the wall behind the insulation cools down less.

But the document makes a warning that matters here. For a home it assumes 55% indoor humidity by default, and warns that by the sea the real figure “may be much higher”, with condensation that, using the standard value, “would not be detected”. A flat near the sea, poorly ventilated and with washing hung up indoors in winter, is exactly that case. There it is worth doing the calculation with the home’s real humidity, or simply fitting the barrier.

Inside or outside?

Inside is the job that depends only on you: one room, one flat, no scaffolding and nobody else’s agreement. It is quicker and cheaper, and it is the sensible answer when the facade belongs to the community and is not going to be touched, or when the problem is a single wall.

Outside is the job that solves the whole building, and it is almost always a matter for the community. What majority the owners’ meeting needs to approve it is on our communities of owners page. If both are possible, insulating from the outside wins on almost everything that matters; if only the inside is possible, done properly, with its calculation or its barrier, it insulates too.

Frequently asked questions

Insulating paint, with figures: how much a thermal paint really insulates.

Sources

• Placo (Saint-Gobain), ficha técnica de «Placomur 1.90 13 + 60 L32» (technical data sheet, April 2026).
• Placo (Saint-Gobain), ficha técnica de «Doublissimo E 1.90 13 + 60» (technical data sheet, April 2026).
• Código Técnico de la Edificación, Documento de Apoyo DA DB-HE/2, «Comprobación de limitación de condensaciones superficiales e intersticiales en los cerramientos» (supporting document on checking surface and interstitial condensation, October 2013).

Quotations from Spanish-language documents are our own translations.